The present invention relates to a nickel-based alloy for producing, by casting, components which have solidified in single crystal form.
Alloys of this type belong to a group of what are known as superalloys, which can be used at high temperatures and under high mechanical stresses and are therefore used in particular as turbine blade materials in gas turbines.
The future generation of aircraft engines with a high bypass ratio and a high-speed low-pressure turbine promises significant improvements in terms of fuel consumption and emissions. The weight of the engine, its size, and the maintenance costs are also current added-value parameters. Engines with a high bypass ratio are equipped with a reduction gear which is connected between the fan on one side and the low-pressure compressor and the low-pressure turbine on the other side. The gear allows the fan to be operated in the optimum range at low rotational speeds and offers the potential for operating the compressor and the turbine at higher rotational speeds and therefore higher pressure ratios than with conventional turbines. However, the higher circumferential speeds also increase the mechanical loads on the blades and discs of the low-pressure turbine.
Second and third generation Ni-based alloys for single-crystal components contain approximately 3% by weight or 6% by weight, respectively, of the refractory element rhenium and have better creep properties than alloys belonging to the first generation without any Re. The refractory element Re has different effects on the properties of superalloys. Re has a large atom radius, and consequently diffuses very slowly and segregates in the matrix. In addition to the effect of solid-solution hardening of the matrix, the rhenium atoms tend to form clusters, which prevent a dislocation motion.
Tungsten makes a considerable contribution to the solid-solution strengthening. The W content influences the distribution of the Re in the matrix and the γ′ precipitation phase.
The high melting point and the low diffusion coefficient of both Re and W lead to an increase in the solidus temperature of the superalloys. Furthermore, the morphology change of the precipitation phase γ′ is delayed under load.
Although the alloying element tantalum (Ta) contributes to the solid-solution strengthening and improves the cyclic oxidation behavior, it is primarily added to W-containing and Re-containing Ni-based alloys to counteract the formation of what are known as freckles during directional solidification.
Negative properties of Ta are a considerable increase in the density; it promotes the undesirable formation of TCP phases; and it increases the γ′ solution annealing temperature.
The increase in the creep rupture strength is associated with a simultaneous increase in the density to 9 g cm−3 for certain alloys containing 6% by weight of Re. In Re-free alloys, the density can be reduced to 8 g cm−3. Ni-based alloys with a high relative density are, however, only of limited suitability for use in modern, high-speed aircraft turbines.
An Re-free superalloy with a low density is known, for example, from U.S. Pat. No. 4,721,540. The trade name of this material is “CMSX-6”. Apart from the mechanical advantage of a relatively low density of 7.98 g cm−3, however, this alloy also has drawbacks, such as a narrow heat treatment window and strong tendency to recrystallize.
Single-crystal castings, the alloy of which contains 0 to 8% by weight of rhenium, 3 to 10% by weight of tungsten and, inter alia, magnesium or calcium for increasing the resistance to oxidation, are known from international publication WO 93/24683. In a specific alloy composition, the Re content is to be 2.8 to 3.2% by weight, the W content is to be 5.6 to 6.2% by weight. Since rhenium and tungsten are heavy metals, this entails a relatively high component density, in particular if the upper limits of 8% by weight of rhenium and 10% by weight of tungsten are used. In addition, rhenium is a very expensive element, which has noticeable effects on the price of the components. The lower limit for Re is given in this document as 0% by weight. Although small quantities of Re reduce the weight and price, they lead to a considerable deterioration in important materials properties.
In view of these drawbacks, an object of the present invention is to provide nickel-based alloys for producing, by casting, components which have solidified in single crystal form, which alloys, through optimization of the rhenium and tungsten contents, allow particularly favourable materials and thus component properties, such as low density, high mechanical strength including low tendency to creep and high thermal stability, to be achieved. Further, it is necessary for the alloy to be easy to cast and to have favourable heat treatment properties.